This invention pertains to the catalytic hydroconversion of special heavy petroleum feedstocks which contain asphaltenes and have Ramsbottom carbon residues (RCR) exceeding about 10 W %, to produce lower boiling hydrocarbon liquid products, and pertains particularly to such process using selective reaction conditions including temperature below about 835.degree. F.
High catalytic hydroconversion operations on heavy petroleum feedstocks, such as achieving more than about 75 V % conversion to produce lower boiling hydrocarbon liquids and gases, are usually carried out in a reaction temperature range of 830.degree. to 860.degree. F. and within a relatively high space velocity range of about 0.8 to 1.2 V.sub.f /Hr/V.sub.r, in order to minimize reactor volume and associated costs. This type of conversion operation has been found useful for many heavy petroleum feedstocks to produce lower-boiling liquids and gases. However, some special heavy petroleum feedstocks exist which have high carbon, as indicated by Ramsbottom carbon residues of 15-35 W %, such as Cold Lake and Lloydminster crudes from Canada and Orinoco tar from Venezuela, which have special characteristics and for which these normal hydroconversion reaction conditions cannot be used, because it has been found that coking of the catalyst bed occurs which makes the process inoperable. The reason for such severe coking is due to the precipitation of asphaltenes because of the imbalance in concentration between asphaltenes and solvent. It has been observed that although other petroleum feedstocks may contain similar amounts of Ramsbottom carbon residue (RCR) in range of 14-26 W %, they do not present the same operating difficulties as the Cold Lake type materials, which have RCR of only about 23 W %.
Prior art hydroconversion processes for petroleum feeds have not provided a satisfactory solution to this problem of processing such special heavy feedstocks, in that it has not disclosed specific ranges of operating conditions suitable for successful hydroconversion operations without resorting to using a diluent oil mixed with the feed. For example, U.S. Pat. No. 3,725,247 to Johnson et al discloses a catalytic process for hydroconversion of heavy oil feedstocks containing substantial asphaltenes at operating conditions within the range of 750.degree.-850.degree. F. temperature and 1000-3000 psig hydrogen pressure, by using a diluent oil and limiting the percentage conversion achieved based on not exceeding a critical heptane insoluble number range. But it does not disclose a combination of moderate reaction temperatures and low space velocity conditions needed for successful hydroconversion operations on such feeds. Also, U.S. Pat. No. 3,948,756 to Wolk et al. discloses a process for desulfurizing residual oils containing high asphaltenes by catalytically converting the asphaltenes and then desulfurizing the treated material. This approach uses relatively mild reaction conditions of 720.degree.-780.degree. F. temperature, 1500-2400 psig hydrogen partial pressure, and liquid space velocity of 0.3-1.0 V.sub.f /Hr/V.sub.r to convert the asphaltenes and provide a product having reduced RCR fo subsequent coking operations, so as to make less coke and more liquid product. However, such reaction conditions were found to be unsatisfactory for hydroconversion processing of certain heavy petroleum feedstocks, such as the Cold Lake and Lloydminster materials.
To carry out successful hydroconversion operations with these special kinds of petroleum feedstocks, a special range of reactor operating conditions has been developed which preferentially hydrocracks the asphaltenes with respect to nonasphaltene resids. These conditions substantially prevent coking of the catalyst bed and provide long term continuous operations without using a diluent oil blended with the feed.